Ultrasonic sensors use an ultrasonic transducer to convert an electrical excitation signal to an ultrasonic pulse. Based on the mechanical inertia of the ultrasonic transducer, an ultrasonic transducer continues to oscillate even after the end of an electrical excitation signal, and emits an ultrasonic pulse that is extended compared to the electrical excitation. This additional duration is denoted as post-pulse oscillation time.
Ultrasonic transducers are also used to receive echos of ultrasonic pulses. Since an echo cannot be distinguished from a post-pulse oscillation, no electrical signal is evaluated during the post-pulse oscillation time.
The post-pulse oscillation time is subject to a multitude of influences, such as contamination, aging, icing in winter, material fatigue, damage by broken rock, later paint work.
A method is discussed in Japanese patent document JP 2003-248050 which newly determines the post-pulse oscillation time of the ultrasonic transducers in a learning mode, during the operation of an ultrasonic sensor. From the post-pulse oscillation response ascertained, one is able to conclude that there was a malfunction.